The present invention relates to a process for the removal of ammonium from cation-containing aqueous liquids, especially fermentation broths, through a transport membrane and selective elimination on the acceptor side, and to an apparatus suitable therefor.
In many fermentation processes, ammonium ions inhibit growth and productivity. This is particularly important in the cultivation of mammalian cells. It is therefore expedient to reduce the ammonium concentration in fermentation processes in order to achieve higher biomass concentrations and higher productivities.
A process for reducing ammonium concentration in such liquids has already been developed and is based on the equilibrium between ammonium and ammonia in such culture liquids, the content of the latter therein being a few percent (Wissenschaftlicher Ergebnisbericht der GBF 1985, pages 20-22). In this process, the ammonia is removed through a porous membrane (polypropylene tube) by diffusion and is taken up by an acid medium on the secondary side of the membrane, and the ammonium ions formed thereby are removed by ion exchange. There is also a report on this procedure by Soeters et al in Advances in Ammonia Metabolism and Hepatic Encephalopathy 69: 534-42 (1988).
Busse et al describe, in J. Hepatology 1 to Vol. 4, p. 10 (1987), the removal of ammonium by means of a lipid hollow-fiber membrane reactor. In this case, ammonium ions on the donor side diffuse as ammonia through a lipophilic hollow-fiber membrane and are converted on the acceptor side either by a pH shift as above or enzymatically with .alpha.-ketoglutarate and NADH into glutamic acid, and thus removed from the transport equilibrium.
T. Pultar et al, DECHEMA BIOTECHNOLOGY CONFERENCES 3--VCH VERLAGSGESELLSCHAFT 1989, pp. 567-71, mention the removal of ammonium ions from culture liquids by adduct formation between ammonium ions and calcium phosphate, but this is described as imperfect. For this reason, the diffusion outlined above, of ammonia through a porous membrane whose acceptor side is flushed with acid solution, is said to be favored.
Another process for reducing the ammonium concentration is described by M. Iio et al in R. Murakami (eds.) GROWTH AND DIFFERENTIATION OF CELLS IN DEFINED ENVIRONMENT, Springer-Verlag, New York 1984, pp. 437-42. The ammonium is absorbed by a suspension of hydrothermally-generated aluminum silicate ZCP-50 in a dialysis tube.
T. E. Hassell et al mention in Spier and Griffiths (eds.) MODERN APPROACHES TO ANIMAL CELL TECHNOLOGY, Butterworths, London 1987, p. 245-63, the reduction in the level of ammonium by replacing glutamine by glutamate or 2-oxo-glutarate.
The above mentioned techniques for reducing the ammonium concentration in cell culture liquids do not appear to be entirely satisfactory. In particular the removal by diffusion of ammonia, which is substantially favored, may give rise to problems over lengthy periods and as a consequence of a change or blockage in the pore structure. Furthermore, the transport rates through the membrane which can be achieved are low, so that very large membrane areas are required.